1. Field of the Invention
The present invention relates to a magnetic memory element, a magnetic memory including the magnetic memory element, and a method for driving a magnetic memory.
2. Related Art
Magnetic random access memories (MRAMs, hereinafter also referred to simply as magnetic memories) utilizing ferromagnetic materials are expected as nonvolatile memories that has nonvolatility, high-speed operability, large capacities, and low power consumptions. Such a magnetic memory has a structure that includes memory cells each having a “tunneling magneto-resistive effect element (TMR element)” as a magnetic memory element. A TMR element is formed with a sandwich structure film that has one dielectric layer (a tunnel barrier layer) interposed between two ferromagnetic layers. In such a TMR element, a current is applied perpendicularly to the film plane, so as to utilize a tunneling current.
In a conventional MRAM, however, magnetic recording is performed by reversing the magnetization direction of a magnetic layer (the magnetic memory layer) of a magnetic memory element by virtue of a local current magnetic field induced through the line provided above the magnetic memory element. Therefore, the value of the current that can be applied to the line becomes smaller as the device size becomes smaller, and induction of a sufficient current magnetic field becomes difficult. Also, the size of a current magnetic field required for recording information in a magnetic memory element becomes larger, as the device size becomes smaller. Therefore, it is predicted that the principles of a MRAM performing a write utilizing a local current magnetic field induced through a line will reach the limit in the 126 Mbits to 256 Mbits generation.
As a means to perform a write through a magnetization reversal with a lower current, magnetic memories that utilize magnetization reversals by spin injections have been attracting attention (see U.S. Pat. No. 6,256,223, for example). A magnetization reversal by a spin injection is caused by injecting spin-polarized electrons having passing through one of the magnetic layers (the magnetization reference layer) into the other magnetic layer (the magnetic memory layer) in a magnetic memory element, so as to induce a magnetization reversal in the other magnetic layer (the magnetic memory layer). By the method of reversing magnetization by a spin injection, the current required for storing information is determined by the current density of the current flowing in the film thickness direction of the magnetic memory layer. Accordingly, the current required for storing information becomes lower, as the device size becomes smaller.
Furthermore, in a magnetization reversal by a spin injection, the current density required for a magnetization reversal hardly becomes higher, though the device size becomes smaller. Accordingly, writes can be performed with higher efficiency than writes utilizing a current magnetic field.
In a conventional spin-injection MRAM, current paths running in the same direction are used for both write and read. Therefore, spins are injected into the magnetic memory layer of each magnetic memory element during a read operation. The spins injected into the magnetic memory layer impart a torque to the spins in the magnetic memory layer. As a result, the spins in the magnetic memory layer are put in an excited state in terms of energy. Since the spins excited in terms of energy has lower resistance to thermal disturbances, information is inadvertently written in the magnetic memory layer during a read operation, and it is very difficult to maintain the stored information over a long period of time.
To counter this problem, the resistance to thermal disturbances is improved by increasing the memory holding energy of each magnetic memory layer, and an inadvertent write during a read operation is prevented. However, an increase in the memory holding energy of each magnetic memory layer leads to an increase in the write current density, which results in another problem.
It has also been suggested that the ratio of the read current to the write current should be increased. More specifically, the read current is reduced, and the write current is increased, so as to prevent an inadvertent write during a read operation. However, the lower limit of the read current is determined by the sensitivity of the sense amplifier, and the upper limit of the write current is determined by the breakdown voltage of the tunnel barrier layer of each magnetic memory element. Therefore, there is a limit to the increase in the difference between the read current and the write current.
Also, to increase the ratio of the read current to the write current where the lower limit of the read current and the upper limit of the write current are fixed, the variations of the read current value and the write current value are reduced. However, it is very difficult to reduce the variations, since the variations in spin-injection elements become wider as the capacity becomes larger.